An electrochemical accumulator usually has a nominal voltage of the following orders of magnitude:                1.2 V for NiMH type batteries,        3.3 V for lithium-ion iron phosphate or LiFePO4, technology,        4.2 V for cobalt-oxide-based lithium ion type technology.        
These nominal voltages are too low for the requirements of most systems that are to be powered. To obtain the appropriate level of voltage, several accumulators are placed in series. To obtain high values of power and capacitance, several accumulators are placed in parallel. The number of stages (number of accumulators in series) and the number of accumulators in parallel in each stage varies according to the voltage, the current and the capacitance desired for the battery. The association of several accumulators is called a “battery of accumulators.”
Such batteries are used, for example, in vehicles to drive an alternating-current electric motor by means of an inverter. Such batteries also have a high capacity in order to favor the autonomy of the vehicle in electric mode. Typically, an electric vehicle uses a battery of accumulators, the nominal voltage of which is on the order of 400V, with a peak current of 200 A and a capacitance of 20 kWh.
Because of their high energy density, lithium-ion electrochemical accumulators are often used for such vehicles. The lithium-ion iron phosphate (LiFePO4) type battery technologies are undergoing major developments because of the intrinsically high level of security. This is achieved to the detriment of density of energy storage, which is somewhat lagging.
It sometimes proves to be necessary to separate the battery from the vehicle, for example to carry out maintenance operations. To this end, the vehicle is provided with an isolating circuit comprising selector switches capable of cutting off the direct current under full voltage, for example vacuum contactors. The isolating circuit is generally external to the battery and designed so that the state of isolation between the battery and the high voltage bus of the vehicle is visually controllable. The terminals of the battery are then accessible to operators. When the isolating circuit is integrated into the battery, it generally also comprises additional terminals that are intended for powering certain peripherals and therefore remain accessible to operators.
However, even when the isolating circuit opens the connection between the battery and the high voltage bus of the vehicle, the totality of the voltage of the battery remains applied to its terminals. Therefore, even at rest, during maintenance operations, the battery presents risks of electrocution for the operators.
Besides, water can infiltrate into the vehicle up to the batteries. The terminals can then be immersed in the water. The resulting exposure of water to the voltage at the battery terminals to the water induces risks of electrocution or hydrolysis of water, with a release of hydrogen and thus a risk of explosion. Owing to the requirement of cooling the battery, the battery cannot be housed as hermetically as would be required to overcome this risk.
Besides, the batteries generally must be put out of service as soon as possible when there is a failure of an accumulator. Indeed, the alternating voltage generated by the inverter depends on the direct voltage that the battery applies to it. As a consequence, a sudden drop in the voltage of the battery can induce problems of operation in the electric motor. The vehicle must then be immobilized and the battery insulated from the high voltage bus by means of the isolating circuit.
A prior-art solution for certain electrical/thermal hybrid vehicles resolves the problem of variation in voltage applied to the inverter in the event of failure of an accumulator. Such a solution comprises a converter between the battery and the inverter. The converter is a voltage step-up device when the battery powers the motor inverter, and a step-down device during the phases of recovering energy, when the motor inverter recharges the battery. Since such a converter works only very briefly in a hybrid vehicle, its size, the sizing of its cooling system and its cost can be minimized.
However, for a vehicle with purely electric drive, this converter has to work with a far higher cyclic ratio. This induces a size, a sizing of the cooling system, and a cost that cause handicaps.